THE OEDER PRIMATES 7
The hallux and pollex are opposable to the other digits and the tail may be an important prehensile organ.
With the exception of Man all Primates have a covering of hair, and the direction of the hair slope presents features of interest from the phylogenetic point of view. Some species have tactile hairs or vibrissse, which are also known as sinus hairs, from the presence of blood-sinuses round their enlarged root-bulbs. They gradually diminish from the lowest Primates up to the highest, and their gradual disappearance may be correlated with the increasing importance of the hand as a sensory organ.
The skin of the palms and soles has a more or less complete set of pads, which form part of the equipment for an arboreal life. In the lower Primates there are pads on the tips of the digits, over the heads of the metacarpals and metatarsals and on the proximal part of the palms and soles. In the Anthropoid Apes they are reduced to traces of the apical series. Besides these pads there may be small elevations of the epidermis.
The skin of the palms and soles is traversed by fine ridges, called papillary ridges, which are arranged in different ways in different Primates. They consist of thickenings of the epidermis over rows of derrnic papillae, and they lodge the orifices of the sweat glands. Their function is twofold—they help to make the grip secure; and they are sensory, for the derrnic papillae contain special sense organs. They become increasingly numerous from the lower Primates up to the highest, because of the progressive increase in the use of the hands as sensory organs. There is, therefore, an inverse ratio between the vibrissse and papillary ridges.